Advances in technology together with an increased understanding of physiological functions has led to the development of a variety of devices which may be implanted into the body to assist or to perform specific functions. Cardiac pacemakers, defibrillators, the Jarvis heart and insulin pumps are just a few examples of these implantable devices. Generally, implantable devices are usually comprised of a power source coupled with electrical and/or mechanical components necessary to perform the desired function. The power source and the other components may require replacement or repair over the life of the patient. Therefore, many implantable devices provide a mechanism for non-invasively providing vital information regarding the device's performance. In this manner, it may be determined without surgery whether the device is in need of repair or replacement or whether the device is approaching a threshold thereby indicating the need for replacement or repair.
A number of cardiac pacemakers available on the market today are capable of being both programmed and evaluated non-invasively. These pacemakers include, for example, the Model 402B Multicor.RTM. II manufactured by Cordis Corporation of Miami, Fla., the Quantum.RTM. Model 254-09 manufactured by Intermedics, Inc. of Freeport, Tex., the Chorus DDD manufactured by ELA Medical, Inc. of Minnetonka, Minn., and the DDD and Genesis.RTM. pacemakers from Pacesetter, Inc. The Ventak.RTM. P Mode 1600 automatic implantable cardioverter defibrillator is another example of an implantable device providing remote programming and evaluation capabilities. These devices typically include an RF transceiver to communicate with an external user interface system, which includes a "programming wand." The external system, such as the Pacesetter.RTM. APS-II Model 3000 Programmer with Model 3030 Function Pack, available from Pacesetter Systems, Inc. of Sylmar Calif., includes controls to allow physician or medical technician program the diagnostic functions of the device and evaluate its operating parameters. The types of information which may be telemetered from, for example, a pacemaker to such an external system include the device's model number, serial number, mode in which the pacemaker is programmed, magnet rate, lead impedance, and electrode/lead information such as the type of electrode implanted in the patient. Also, battery life, one of the vital characteristics affecting the performance of the pacemaker, may also be telemetered. Similarly, defibrillators such as the Ventak.RTM. P AICD.TM. Model 1600 manufactured by Cardiac Pacemakers, Inc. of St. Paul, Minn., also are capable of telemetering performance information to such a programming system.
Some types of problems are not readily discernable with the pacemakers and the defibrillators currently available. Specifically, with regard to an electrode connected to such devices, little information, with the exception of type of electrode used, is available through an external programming system. Various types of problems can occur with such electrodes including lead fracture, lead displacement, body reaction to the lead interface, migration of the lead through body tissue, unsatisfactory electrode position and faulty connection with the implantable device. For example, the electrode may be improperly fastened to the pacemaker resulting in an ohmic or loose junction or, after the electrode is implanted, it may rub against a bone within the patient's body and strip the electrode's insulation. Thus, it is desireable to develop an implantable device having an electrode which is capable of providing information about the integrity of the electrode in a non-invasive manner both at the time the device is implanted and throughout the time the electrode remains implanted.
One known method used to attempt to determine the integrity of an implanted electrode is an X-ray radiograph. However, X-ray radiographs are not adequate for integrity testing as they are unable to provide information about the connection between the device and the electrode or the condition of the electrode in a reliable manner. For example, an X-ray radiograph may, in some cases, indicate that a fault is located at the point where the electrode is connected to the pacemaker. However, the performance of other tests or examination of the electrode during a surgical proceeding may reveal a pseudofracture, i.e., no actual fracture is present, such as is caused by the excessive tightening of a suture at that point. Thus, X-ray radiographs can lead to unnecessary surgery intended to correct a non-existent problem. Therefore, it is desireable to determine the integrity of an implanted electrode through the performance of a single, reliable test. Additionally, such a test should not be as susceptible to interpretation or to patient conditions as is X-ray radiography.
Various pacemakers and defibrillators can accommodate various types of electrodes. Generally, there are two types of electrodes. Unipolar electrodes are defined as those in which the anode is the case of the planted device and the cathode is the electrical lead. Bipolar electrodes are those in which the anode is the proximal lead electrode and the cathode is the lead electrode. Examples of bipolar electrodes include the VS1 Bipolar Tined Electrode manufactured by Oscor Medical, Inc. of Palm Harbor, Fla. Some devices such as defibrillators require the utilization of bipolar electrodes so that the defibrillator may deliver shocks to the heart as well as simultaneously monitor the heart's function. Thus, it is desireable to develop an integrity testing system which may be used to test the integrity of both unipolar and bipolar electrodes.
Electrodes for many applications are insulated so as to avoid affecting or being affected by the surrounding tissue. However, the electrodes can deteriorate over time. Thus, it is desirable to develop an integrity testing system for an implanted electrode which is not significantly affected by the natural deterioration of the electrode.
It is also desirable to develop a method for testing the integrity of the electrode which does not interfere with the normal operation of the implanted device. A pacemaker, for example, must send pulses to the heart at a specified rate such as 60 pulses per second. For the pacemaker to continue to operate during the testing procedure, the integrity test must be performed without interfering with those pulses.
Additionally, most implantable devices require little power to operate. Because little power is required to perform the desired function of the device, a battery may be utilized for a lengthy period of time without requiring replacement. Therefore, it is desirable to develop an integrity testing system which does not require significant power to operate so as to avoid reducing the life of the battery used in the device.
Time domain reflectometers, such as the 1502C Metallic Time Domain Reflectometer manufactured by Tektronix, Inc. of Beaverton, Oreg., are used to test the integrity of cable such as co-axial cables. For such integrity testing, time domain reflectometers send electrical pulses down the cable and detect any reflections made by any discontinuities in the cable. Specifically, time domain reflectometers send out successive pulses and measure the respective reflected pulses at times corresponding to points along the cable. Measurements are provided in terms of voltage versus time which can then be converted to resistance over the length of the cable. Time domain reflectometers can locate shorts, opens, defects in the shield of the cable, foreign substances in the cable, kinks, and more. Generally, only one parameter is required for the proper operation of the time domain reflectometer in determining the integrity of a cable. That parameter is the velocity of propagation or the speed of the signal down the cable which varies for different cable dielectric materials. Time domain reflectometers may operate on either a closed or an open circuit. For an open circuit the signal continues to be reflected through the air (or other medium) and returns to the instrument. In general, variations in the resistance measured by the time domain reflectometer indicates a fault such as a bad connection, the stripping of insulation, pressure on the cable, or a break in the cable.
Time domain reflectometry has been used for a variety of applications. In U.S. Pat. No. 4,466,288, time domain reflectometry is used to evaluate vibrations. The level of fluid in a vessel may be determined by time domain reflectometry as disclosed in U.S. Pat. No. 3,922,914. Also, the constituents of a multi-phased fluid system have been evaluated as disclosed in U.S. Pat. No. 4,786,857.
In addition, time domain reflectometry has been used for optical systems as well. For example, optical time domain reflectometers, such as that disclosed in U.S. Pat. No. 4,960,989, may be used to determine the tip location of a consumable electrode within an electric furnace as disclosed in U.S. Pat. No. 4,843,234. Similarly, optical time domain reflectometry is used in U.S. Pat. No. 5,033,826 to determine which surface of a photographic lens is impairing transmissivity.
It is desirable to provide a method and device using time domain reflectometry to determine the integrity of an implanted electrode to thereby alert the cardiologist or the technician of a potential or existing problem associated with the electrode. As indicated above, time domain reflectometry may be used with both unipolar or bipolar electrodes. The velocity of propagation of any electrode is necessary for time domain reflectometry measurements. Such information could be stored in the implanted device.
It is also desirable to provide a method of analyzing the integrity of the electrode connected to the implantable device. Such analysis could be completed in a programmer such as those used for the analysis of presently available data.